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Duripans of Southwestern Idaho: Polygenesis during the Quaternary Deduced through Micromorphology

USDA-ARS, Ecology of Temperate Desert Rangelands Unit, 920 Valley Road, Reno, NV 89512

B. Cochran

Geology Dep., Walla Walla Community College, Clarkston, WA 99403

M. A. Fosberg

College of Agriculture, Univ. of Idaho, Moscow, ID 83844

*Corresponding author (blank{at}scs.unr.edu).

ABSTRACT

There is uncertainty regarding the pedogenesis of duripans on the Owyhee Plateau of southwestern Idaho. Micromorphology was used to deduce the role of polygenesis in their formation. Data suggest that during the Pleistocene, duripans were cyclically subaerially exposed, then recovered by eolian dust. Two lines of evidence indicate past duripan exposure. First, sinuous laminar fabrics with lenticular voids, which occur within most duripan plates, are similar to the fabrics produced in arid regions by the action of lichens and cyanobacteria at the soil surface. Second, a laterally extensive air-fall tephra layer in the middle of a duripan plate was deposited at one sampling location, indicating the duripan or protoduripan was exposed at the ground surface. A suggested model to explain the data is as follows: (i) during cold stages of the Quaternary, periglacial processes, possibly assisted by eolian deflation, caused the truncation of loose soil material overlying the duripan; (ii) cryptogamic organisms then colonized the exposed duripan surface and produced distinctive fabrics; (iii) in response to the increased production and delivery of eolian dust during interglacials, the duripan was recovered. Pedogenesis during interglacial times resulted in precipitation of CaCO₃, thereby thickening the upward-developing duripan and preserving micromorphological signatures of subaerial exposure. At higher elevations in this region, large-scale removal of unconsolidated soil may have occurred during full glacial stages of the middle and earlier Pleistocene. Maximal development of duripans may require polygenesis.

Received for publication January 28, 1997.

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